Vehicle engine systems may include various vacuum actuators, such as vehicle brakes, that utilize vacuum as an actuation force. The vacuum is typically supplied by the engine through a connection to the intake manifold, which is at sub-barometric pressure when the intake throttle is partially closed and regulating the airflow into the engine. In some examples, engine-driven or electrically-driven vacuum pumps may be used to supplement intake manifold vacuum during conditions (e.g., during an engine cold-start and warm-up) when intake manifold vacuum does not provide sufficient vacuum for operating all the various vacuum actuators. However, engine-driven vacuum pumps may disadvantageously reduce fuel economy, while electrically-driven vacuum pumps may lack durability while being expensive, heavy, and noisy. In still other examples, ejectors positioned at different locations in the engine system may be used to provide at least a portion of the vacuum used by the actuators. In particular, flow of air and/or exhaust gas through the ejector may be harnessed to generate vacuum for use by the vacuum actuators. However, since vacuum generation at an ejector is related to flow through the ejector, changes in air flow or exhaust flow, such as during boosted operating conditions or during exhaust gas recirculation, may affect vacuum generation at the ejector.
The inventors have recognized the issues with these options and offer systems and methods for more reliable vacuum generation at different engine operating conditions, with the further advantage of expediting catalyst warming. In one embodiment, a method for an engine includes, while recirculating an amount of exhaust gas to an engine intake, closing a post-catalyst exhaust throttle to increase vacuum generation at an exhaust ejector, and closing an EGR valve to maintain the amount of exhaust gas recirculation.
The presented approach may offer several advantages. For example, rapid catalyst heating may be attained. By rapidly heating the catalyst, exhaust emissions during engine cold starts may be reduced. Additionally, vacuum may be generated in copious amounts during the very condition (catalyst heating) when it is less available via the intake manifold. This is accomplished by directing exhaust through the ejector, thus reducing the need for engine-driven or electrically-driven vacuum pumps to supplement intake manifold vacuum. Further, by adjusting the EGR valve in concert with the exhaust throttle, vacuum may be generated in the presence of EGR flow, as well as while exhaust flow through an EGR passage fluctuates. At the same time, a desired EGR rate and engine dilution can be maintained so that engine performance is not degraded.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.